They do nothing. Take it from someone who has put one on every vehicle they've owned. JLT claimed a 30rwhp gain on 05-09 mustang gt's, I felt NOTHING. Most people claim these gains to justify to themselves the money they spent. You can spend $400 on a kit, or get the same result for much less. You achive gains from tunes, gears, cams, heads, torque conveters, superchargers, turbos, LONG TUBE headers, Take your pick. There, I said it.